The present invention relates generally to optical apparatus and, more particularly, to an off-axis conic reflective optical apparatus such as a collimator.
Collimators are optical devices, belonging to a family of devices known as collimators or telescopes, which include products called beam expanders and condensers. Such devices increase or decrease the size of and change the divergence or angular characteristics of a light beam passed therethrough. Such devices also have other characteristics and functions known to those skilled in the trade. Collimators are used in various applications such as laser interferometers, devices employing a tracking or marker beam, laser-equipped machine tool industry, and the like.
Collimators may be constructed of transmissive optics such as lenses through which the light beam is passed. Use of transmissive collimators with lasers with power levels higher than about three kilowatts/m2 becomes increasingly problematic due to limits on energy density that transmissive optic materials can withstand and due to a phenomenon called thermal lending. Thermal lensing is the distortion of an optical component caused by heat absorption typically from an input beam of light. The distortion can influence the divergence and mode quality of the beam passing through or reflecting from the optical component and cause detrimental shifts of focus position and diameter. Lenses are more sensitive to thermal drift due to changes in the optic's index of refraction. In addition, focusing or collimating the beam for space or other in-vacuum applications is complicated by the change in the relative index of refraction between vacuum and in-air.
Collimators may also be constructed of reflective optics, combinations of flat and shaped mirrors, such that the light beam is reflected from these optical elements. Reflective optical materials can withstand greater energy densities without damage and thermal lensing is not as severe in reflective optics as in transmissive optics. Thus reflective collimators are more suitably used in high power laser applications or in applications where even slight thermal lensing is undesirable or unacceptable. Some known reflective collimators consist of several (e.g., four) reflective optics. By adjusting the distance between specific optics the divergence of the output beam can be varied. The mounting and alignment of optical components in a reflective collimator, however, can be complicated and can introduce inaccuracies. It is advantageous to reduce the number of optical components used in the beam delivery system. Each optic adds complexity to the task of aligning a beam path, is a possible source of beam distortion and absorbs some energy from the beam reducing overall system efficiency. Optical elements degrade with use. Each optical element adds to the overall cost of system maintenance.
Reflective collimators typically employ parabolic reflectors, although other conic sections such as elliptical reflectors have been used. Existing assemblies utilizing parabolic or other conic reflective surfaces often involve complex packaging, adjustment, alignment, and mounting techniques. Moreover, on-axis parabolic collimators produce beams that are corrupted by shadows due to the existence of a hole in the center for the exit (or entrance) pupil. Existing off-axis collimator assemblies are even more complex than on-axis collimator assemblies, and involve additional mounting and alignment components.